It was mid-morning on Sunday when Algeria's special forces - known as ''the Ninjas'' - launched their final, fatal assault.

The result was perhaps inevitable - a terrible bloodbath on the ground, ending in the deaths of seven foreign hostages as well as 11 of the kidnappers.

The Algerian military had already proved itself trigger-happy when 48 hours earlier - and without the knowledge of Britain or any other Western government - army helicopters had opened fire on a fleet of 4x4s speeding through the Tigantourine gas facility, killing terrorists and hostages indiscriminately.

Hostages at the Tigantourine gas facility in Algeria. Photo: Reuters

By yesterday morning, the rump of the heavily-armed Islamist militants had retreated from the accommodation blocks to the main gas complex itself more than three kilometres away. Reports suggested that they had holed up in a machine room with a reported 23 hostages being used as human shields.

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The Algerian authorities had refused to negotiate with the terrorist group, known variously as the Masked Brigade or Signers in Blood. Its leader on the ground was named as Abdul Rahman ''the Nigerien'', also known as Abu Dujana.

Abdul Rahman had led about 40 fighters over the border from either Libya or Niger to the outskirts of the plant on Wednesday morning, where the onslaught began.

Seventy-two hours later, Abdul Rahman and 10 of his men had barricaded themselves into the machine room with a cache of weapons. They had with them hostages from Britain, the US, Germany, Romania and Portugal.

What happened next remained unclear on Sunday night but details can be pieced together from the various sketchy reports.

Some 16 hostages - including two Americans, two Germans and one Portuguese - were freed but a further seven were left dead, either executed in cold blood by the terrorists or else mowed down in the firefight. ''The assault took place mid-morning. Eleven terrorists lost their lives along with the foreign hostages,'' an Algerian security source told AFP news agency. ''We think they were killed in retaliation.''

Official sources told Algeria's al-Watan newspaper, a journal known for its close contact with the country's feared secret services, that the terrorists had tried to set fire to the gas facility's installations on Friday night.

It was a last throw of the dice as they realised their series of demands - for the release of Islamist terrorists in the US, the removal of French troops from neighbouring Mali and a request for safe passage - went ignored.

''We're pressing the Algerians for details on the exact situation,'' British Defence Secretary Philip Hammond told a joint press conference with Leon Panetta, his US counterpart.

The loss of life, he said, was ''appalling and unacceptable and we must be clear that it is the terrorists who bear sole responsibility for it''.

The Algerian response had begun on Thursday, with army attack helicopters, capable of firing 5000 rounds a minute, shooting indiscriminately at seven vehicles, driven by the kidnappers but which also contained about 35 hostages.

The militants, about 15 of them, fired back. In the firefight, four cars were blown up and one crashed. The militants placed ''an explosive cord'', according to one survivor, around their necks and they were told that they would explode if they tried to run away.

Ground troops moved into the complex in the aftermath of that first skirmish, searching through the facility for survivors before attempting to secure the strategically important gas plant. At some point in that operation another of the terrorists' leaders, Abu al-Baraa ''the Algerian'', is believed to have been shot.

The army assault was led by General Othman ''Bachir'' Tartag, 60, the deputy head of the security services. General Tartag is under strict orders to eliminate al-Qaeda in Algeria.

''He has a reputation for brutality, which goes back to the Algerian civil war in the 1990s,'' said one expert.

Mohamed Larbi Zitout, a former Algerian diplomat now living in exile in London, said the secret services, led by General Mohamed ''Toufik'' Mediene, had remained at the heart of power despite changes of government over the past two decades.

''He [Mediene] runs everything with his deputy General Tartag, who is known for being one of the most bloodthirsty men in Algeria,'' said Mr Zitout.

He added: ''They have a state of mind that Algeria must be run by the army. They don't believe that civilians could run the country.''